Thanks, Dwight!
There is the "Use Alpha Channel for transparency" Button in AC 8.1 too, but it isn't working.
I used to render against a nasty-coloured-backgroung before I discovered Alpha-channel
When cleared in Photoshop I ended up with a glowing contour of the building. Great for renderings for "The Master of the Rings" movie.
Thaks God (and Discreet) we don't have this problem in MAX.